1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) pixel circuit, and more particularly, to a technique for driving the pixel circuit that minimizes stress effects of a TFT device that provides current to the OLED.
2. Description of the Prior Art
An organic light emitting diode (OLED) pixel may utilize any of a variety of organic materials that emit light when an electric current is applied thereto. An OLED display comprises a plurality of OLED pixels organized into an array.
One method to achieve a large size and large format OLED display is to use an active matrix thin film transistor (TFT) back plane. A head mount display and even a direct view display for a small mobile application may use polysilicon or crystalline silicon as a back plane. Due to investments in amorphous silicon flat panel technologies, there is interest in using amorphous silicon (a-Si) as opposed to polysilicon (p-Si) or crystalline (c-Si) silicon as a back plane technology to make a larger OLED display. Large area crystalline silicon back planes would not be as cost effective as amorphous or polysilicon.
Amorphous silicon does not have complimentary devices, as are available in polysilicon or crystalline silicon, for two reasons:
(1) only n-channel field effect transistors (NFETs) are available in amorphous silicon flat panel display (FPD) manufacturing due to fewer photolithographic steps, and hence lower costs, as compared to polysilicon and
(2) p-channel field effect transistors (PFETs), although possible to make, exhibit substantially lower mobility or charge transport due to drift (approximately a factor of 5 to 10), and hence lower current drive, than n-channel field effect transistors (NFETs). NFETs have an average mobility approximately 0.5 to 1.0 cm2/V/sec in conventional manufacturing lines.
Due to a manner in which OLEDs are processed, it is not normally possible to drive OLEDs with an NFET configured current source. In conventional active matrix addressing, voltage signals are written into each pixel to control brightness of each pixel. The mobility and the stability characteristics of threshold voltage and mobility of amorphous silicon are suitable for driving twisted nematic liquid crystal, which is electrically similar to a small capacitive load, where a driving voltage is applied with a duty cycle in the range of 0.1% to 0.001%. However, for driving OLEDs requiring continuous current for operation, the amorphous silicon operating voltages are non-zero for a substantially larger percentage of the time, e.g., duty cycles of up to 100%. The higher voltages and continuous current severely stresses the amorphous silicon TFT. In particular, a gate to source voltage stress causes a threshold voltage to vary due to trapped charging and other effects such as creation of defect states and molecular bond breakage at a gate insulator-to-semiconductor interface and in a semiconductor layer of the TFT.
As the TFT""s threshold voltage varies, current though the TFT will vary. As the current varies so does brightness of the OLED since light output of the OLED is proportional to current. A human observer can detect a pixel to pixel light output variation of as little as 1%. A higher level of 5% luminance variation is typically considered to be unacceptable.
FIG. 1 is a schematic of a prior art pixel circuit 100 used in a small a-Si backplane display test vehicle. Circuit 100 includes NFETs Q101 and Q102, a capacitor Cs 110 and an OLED 120.
NFET Q101 and Cs110 store a pixel voltage. A high voltage level on a gate line 125 turns NFET Q101 ON, thus providing a voltage from a data line 130 to Cs110. After a period of time, the gate voltage of NFET Q102 is the same as the voltage on data line 130, and voltage on gate line 125 is set low. NFET Q102 operates as a voltage follower to drive OLED 120. Current through OLED 120 is sourced from a supply voltage Vdd and returned to a supply voltage Vss. As OLED 120 is driven, a threshold voltage (Vt) of NFET Q102 changes with time t. The voltage across OLED 120 is
Vddxe2x88x92Vcsxe2x88x92Vgs(t)xe2x88x92Vss, 
where:
Vcs=voltage across Cs110;
Vgs(t)=voltage gate-to-source of NFET Q102 as function of time t; and
Vss=negative supply voltage or OLED cathode voltage
The current through OLED 120 or NFET Q102 is proportional to (Vgsxe2x88x92Vt)2 because NFET Q102 is biased in its saturation or constant current regime in which the drain to source voltage is equal to or greater than Vgsxe2x88x92Vt. As a result, voltage across OLED 120 and current through OLED 120 changes as the threshold voltage (Vt) of NFET Q102 changes. With different driving histories from pixel to pixel, pixel to pixel current and luminance vary. This is known as pixel differential aging. The threshold variation of NFET Q102, which requires continuous current for operation, is considered unacceptable for many applications. However, the stress of NFET Q102 operating in its saturation regime is less than if NFET Q102 was biased in its linear regime, the drain to source voltage  less than Vgsxe2x88x92Vt.
For use with a-Si TFT back planes, circuit 100 requires relatively low power and voltage since only one NFET, i.e., NFET 102, is connected from power supply Vdd to OLED 120, which is connected to supply voltage Vss. Since OLED 120 current passes through a single NFET, the voltage difference in power supplies Vdd and Vss is kept to a minimum, i.e., a maximum OLED 120 voltage and the drain to source voltage of NFET Q102 for operation just into the saturation regime.
A circuit that is similar to circuit 100 replaces NFET Q101 and NFET Q102 with PFET Q101 and PFET Q102, respectfully, which can be used with polysilicon or crystalline silicon technology. Instead of PFET Q102 operating as a voltage follower, PFET Q102 operates as a current source. PFET Q102""s threshold voltage has an even greater impact on the current into OLED 120 since the current through OLED 120 is proportional to (Vcsxe2x88x92Vt)2 where Vgs=Vcs. If crystalline silicon, which has a high transconductance, is used, then the Vgs voltage would have to be less than Vt in order to produce a current low enough to drive OLED 120 at brightness levels of the order 100/cd/m2 since pixel dimensions are usually very small. Threshold voltage variations in the subthreshold regime have an even greater impact on drain current variations because there is an order of magnitude current change for every 60 millivolt change in threshold voltage, or as dictated by a transistor drain current-gate voltage inverse sub-threshold slope, or approximately 60 mV/decade of current.
To minimize stress effects of a TFT device that provides OLED current, current driving is used to write a voltage stored in a pixel circuit. Sony Corporation, 7-35 Kitashinagawa 6-chome, Shinagawa-ku, Tokyo 141-0001, Japan has shown a polysilicon current mirror pixel in a 13xe2x80x3 diagonal 800xc3x97600 color active matrix OLED (AMOLED) display. The Sony circuit was published by T. Sasaoka et al., xe2x80x9cA 13.0-inch AM-OLED Display with top emitting structure and adaptive current mode programmed pixel circuit (TAC)xe2x80x9d, in 2001 SID International Symposium Digest of Technical Papers, volume XXXII, p384-387. In the Sony circuit, data on its data line is in the form of current rather than voltage. However, the Sony circuit does not correct for threshold variation of an OLED driving transistor.
A four PFET transistor circuit for use with polysilicon was developed by Sarnoff Corporation, 201 Washington Road Princeton, N.J. 08543-5300, as described by R. M. A. Dawson et al., xe2x80x9cThe impact of the transient response of organic light emitting diodes on the design of active matrix OLED displaysxe2x80x9d, in IEDM, p875-878, 1998. The Sarnoff circuit uses a data line current to directly set a current in a transistor that drives an OLED. However, the circuit requires polysilicon and uses two transistors in series between the OLED and a power supply and has a third input control signal that could be used for dark gray scale capability in high resolution displays. The third input control adds complication to the physical design pixel circuit and array design.
An alternative four polysilicon transistor arrangement was developed by Phillips Research, 5656 AA Eindhoven, the Netherlands, as described by T. van de Biggelaar et al, xe2x80x9cPassive and active matrix addressed polymer light emitting diode displaysxe2x80x9d in Flat Panel Display Technology and Display Metrology II of the Proceedings of the SPIE, Vol. 4295 p134-146, 2001. This arrangement eliminates the third input control signal of the Sarnoff circuit, but also uses two transistors in series between the power supply and the OLED. The elimination of the third input does not allow its use in high-resolution displays having dark gray scale capability.
A similar circuit using four amorphous silicon NFET transistors using data line current was published by the University of Michigan, Ann Arbor, Mich. 48109, and more specifically by Yi He et al., xe2x80x9cCurrent-source a-Si:H thin film transistor circuit for active-matrix organic light-emitting displaysxe2x80x9d, in IEEE Electron Device Letters, vol.21, No.12, p590-592, 2000. One limitation of this circuit is that a second transistor is connected in series with an OLED current generating transistor to a power supply. This pixel circuit also would not be used in high-resolution displays having dark gray scale capability.
The present invention provides a method for driving an organic light emitting diode (OLED) pixel circuit. The method includes applying a first signal to a terminal of the OLED when setting a state of the pixel circuit, and applying a second signal to the terminal when viewing the state.
The present invention also provides a driver for an OLED pixel circuit. The driver includes a switch that directs a first signal to a terminal of the OLED when setting a state of the pixel circuit, and that directs a second signal to the terminal when viewing the state.